1. Field of the Invention
The present invention relates to a surge arrester comprising an insulating housing containing a plurality of cylindrical varistor blocks arranged coaxially in a stack, the end surfaces of the varistor blocks being provided with electrodes for electrical series connection of the blocks in the stack, the envelope surfaces being tightly surrounded by annular protection members of the insulating material. The invention is preferably intended for surge arresters comprising zinc oxide varistors.
2. Prior art
Zinc oxide varistors are strongly non-linear varistors which consist of zinc oxide (exceeding 90%) and some other metal oxides which are mixed, shaped and sintered at a high temperature into cylindrical bodies, the envelope surface of which is provided with a thin, electrically insulating (preferably ceramic) coating and the end surfaces with electrodes of a suitable metal (see, e.g., U.S. Pat. No. 4,046,847). Because of the strong non-linearity of these varistors, they are extremely suitable for use in surge arresters, since in that case the spark gaps necessary in conventional surge arresters with silicon carbide varistors can be completely omitted, or alternatively, the number of spark gaps be heavily reduced. The surge arresters built up from zinc oxide varistors usually consist of a plurality of cylindrical varistor blocks stacked in series (possibly together with a small number of spark gaps) in porcelain housings which are hermetically sealed and provided with overpressure relief means in a known manner. The stack or stacks of varistors are usually arranged centrally in the porcelain housing with a free space between the stacks and the housing, so that an overpressure generated during a short-circuit in the surge arrester can be discharged through protective members at the ends of the surge arrester. For surge arresters having several stacks of varistors connected in parallel, electrically and mechanically, metallic guide plates may be used to accomplish the parallel connection and fix the stacks laterally, evenly spaced from each other.
Compared with silicon carbide varistors, zinc oxide varistors have a relatively level current-voltage characteristic. This means that a stack of zinc oxide varistors has a relatively high voltage stress in the longitudinal direction also at relatively small currents. In connection with a fault in a varistor block or bad contact between two adjacent blocks of varistors, glow discharge may arise and a small part of the total flash-over distance between the ends of the stack of varistors be ionized. There is then a certain risk of a total flash-over occurring in the porcelain housing, especially at overvoltage stresses of a long duration (several milliseconds), which particularly occur with applications for high voltage direct current and in connection with alternating voltage upon discharge of long lines or cables.
It is previously known to provide zinc oxide varistor blocks with a protection means for the envelope surface of, for example, silicon rubber (U.S. Pat. No. 4,100,588). Among other things, this protects the blocks from mechanical damage during transport and other handling, and further the heat emission capacity of the blocks is improved by bringing the rubber into contact with part of the internal periphery of the housing. However, this previously known protection means provides little protection against flash-over initiated by glow discharge caused by bad contact between adjacent blocks or a fault in an individual block.